Refrigeration systems as the one defined above normally comprise a compressor rack having variable capacity, a condenser and a number of refrigerated display cases. An example of such a refrigeration system is outlined in FIG. 1. Each display case is typically equipped with a control valve and an evaporator. The control valve serves as ON/OFF valve and as superheat control (expansion) valve and is typically a solenoid valve. When the control valve is a solenoid valve, the superheat is typically controlled by a pulse-width modulation approach. Alternatively, each display case may be equipped with an ON/OFF valve in combination with a thermostatic expansion valve.
The display cases of the refrigeration system are typically controlled according a hysteresis control strategy. In such a control strategy a representative temperature Tdisplay of a display case is measured. This temperature is compared with the predetermined upper, TCutIn, and lower, TCutOut, limits of a temperature band. When Tdisplay is equal to or higher than TCutIn the control valve is activated and starts controlling a flow of refrigerant into the evaporator while maintaining a sufficient superheat, thereby switching the evaporator from an inactive to an active state. By switching the evaporator to an active state, the case is refrigerated. The evaporator continues to be in the active state until the display temperature Tdisplay is equal to or lower than TCutOut. When this is the case, the control valve is turned inactive, whereby it prevents the refrigerant from flowing into the evaporator until the display case temperature reaches TCutIn. Using this control strategy the display case temperature is kept within the temperature band defined by TCutIn and TCutOut with minor over- and undershoots. The overshoots are generally small and they arise because there is a minor time delay from activating the control valve till the refrigerant is evaporated and the refrigeration starts affecting the display case temperature Tdisplay. The undershoots are typically somewhat larger. They arise because the evaporator contains a certain amount of refrigerant (and because of the thermal capacity of the evaporator), when the control valve stops the flow of refrigerant into the evaporator. The temperature (Tdisplay) will continue to drop until the refrigerant in the evaporator has evaporated, and until the temperature of the evaporator equals Tdisplay.
When controlling the display cases according to a hysteresis control strategy, the case temperature Tdisplay cycles with a certain periodicity. Experience shows that the periodicity is nearly independent of the level of the temperature settings and the case type. Experience also shows that the cases tend to synchronize their temperature cycles so that they reach TCutIn almost at the same time, thereby causing the control valves to be activated almost simultaneously. Similarly, TCutOut is also reached by the cases at approximately the same time. This synchronization process is reflected in FIG. 2. This can be explained by the fact that the evaporators absorb more heat from the surrounding air when the suction pressure is relatively low than when the suction pressure is relatively high. Hence, when the majority of evaporators are inactive, thereby causing the suction pressure to be relatively low, the remaining active evaporators are able to drive the temperature down to TCutOut faster. Thereby the active evaporators will ‘catch up’ with the evaporators which are dominating the suction pressure, i.e. the slopes of the temperature curves corresponding to the active evaporators will become steeper. Since the control valves are turned active and inactive almost simultaneously, the synchronization process leads to a fluctuating suction pressure, and even a periodically fluctuating suction pressure.
The suction pressure is normally controlled via a compressor controller by increasing or lowering the number of compressors turned on or off. The compressor controller typically runs the compressors according to a Proportional Integral Derivative (PID) control strategy, often with a deadband compensation. The suction pressure is controlled on the basis of suction pressure measurements done with a pressure sensor at the inlet of the compressor rack. The synchronization initiated pressure fluctuations having the same periodicity as the case temperatures results in frequent turning compressors on and off with the same periodicity as the temperature fluctuations. This results in significant wear on the compressors, as they tend to follow the period of the display cases. The period of the display cases is typically in the order of minutes. This is a great disadvantage.
U.S. Pat. No. 5,460,008 describes a method of controlling a plurality of commonly piped compressors for a refrigeration system having a plurality of refrigeration cases. The method comprises the steps of sensing a suction pressure of the refrigeration system, determining whether the sensed suction pressure is within a predetermined pressure range, and turning compressors ON or OFF in stages until the suction pressure is within the predetermined pressure range. The method also includes the steps of sensing a case temperature for each of the refrigeration cases if the sensed suction pressure is within the predetermined pressure range and determining whether the sensed case temperature is within a predetermined temperature range. The method further includes the steps of turning selectively the load on each of the refrigeration cases ON or OFF when the case temperature is within the predetermined temperature range until the sensed suction pressure is within a predetermined synchronization pressure range.
Thus, in the method described in U.S. Pat. No. 5,460,008 the suction pressure is controlled partly by turning the load on the refrigeration cases ON or OFF, partly by turning the compressors ON or OFF.
EP 0 410 330 describes a method of operating a refrigeration installation, in particular a compound refrigeration installation having at least two compressors connected in parallel. A reference signal for the current cooling conditions at a cooling point is transmitted from each of a number of sensors to a central unit, which accordingly switches on or off the connected compressors. The measured values of temperature sensors as well as the respective coolant suction pressure are used as reference signal and are evaluated in the central unit. Thus, the compressor capacity is controlled on the basis of a measurement of the suction pressure.
However, it is a disadvantage of the method described in U.S. Pat. No. 5,460,008 and the method described in EP 0 410 330 that the load on the refrigeration cases as well as the compressor capacity are controlled on the basis of a measurement of the suction pressure, and that the object in both cases is to control the suction pressure to be within a desired pressure range. Thereby the same object is sought by controlling two different entities on the basis of the same control parameter. This introduces a risk that, in case the suction pressure approaches a limit of the desired range, the control system will attempt to counteract this by means of the refrigeration cases as well as by means of the compressors. The two manners of controlling may thereby either counteract each other or amplify each other, and the result may be that the suction pressure goes out of control. This is in particular a problem when the controlled variable, in this case the suction pressure, does not react instantaneously to a change of the control signal.